This invention pertains generally to the field of power conversion and more particularly to a pulse width modulated switching power supply with linear feedback control.
Compact and efficient power supplies are an increasing concern to users and manufacturers of electronics. Pulse width modulated (PWM) switching power supplies offer both compactness and efficiency in a number of different topologies. Boost and buck PWM switching power supply topologies are efficient, but do not isolate the power input from the power output. Other topologies, such as the flyback, do isolate the power input from the power output by using a transformer. In such topologies, however, feedback from the secondary (power output) side of the transformer is required to adjust the pulse width modulation of the power switch. To properly compensate the feedback from the secondary requires extra components and often involves expensive re-design, depending upon the particular application.
In contrast to a PWM switching power supply, soft-switched converters possess resonant elements to generate sinusoidally varying resonant voltages and/or currents that help reduce switching losses. Notably, in a particular form of soft-switched converter, a resonant transition converter, LC elements coupled to two power switches that turn on and off only at either zero current states or zero voltage states during a sinusoidally varying resonant current or voltage waveform minimizes switching stress and loss. In general, the behavior of these resonant waveforms depends on the values of the inductance and capacitance within the resonant tank as well as values of the DC input and output voltages. Accordingly, considerable research has been conducted on the relationship between these values and the resonant waveforms. Researchers have discovered that to maintain a constant output voltage independent of the output current from such series or parallel resonant converters requires, for example, frequency modulation of the switching elements within the resonant converters. See, e.g., U.S. Pat. Nos. 4,796,173, 5,448466, 4,017,784, 4,727,469, and 4,757,432. However, because of the nonlinear loading within a resonant tank, analysis and design of the feedback control systems for these converters is difficult and cumbersome.
There is a need in the art for an improved PWM switching power supply that combines the simplicity and ease of design provided by a PWM switching power supply yet provides the stress and loss advantages of a resonant converter without requiring adjustment, such as through frequency modulation, of the properties of the resonant converter. Further, there is a need in the art for a PWM switching power supply that isolates the input and outputs through a transformer without requiring feedback from the secondary side of the transformer, thereby easing design and reducing the component count. In addition, there is a need in the art for an improved resonant converter and methods of controlling such converters that avoid the complexity of prior art methods involving frequency control, magnetic flux control, or impedance adjustment of resonant converters. The present invention addresses these needs by providing, in one embodiment, a resonant converter whose output is regulated by a DC input voltage that is in turn is adjusted accordingly by the output current or voltage status.
In accordance with one aspect of the present invention, a power converter comprising a PWM switching power supply coupled to an alternating current tank is provided. The PWM switching boost power supply includes a power switch for regulating an internal voltage output. The alternating current tank comprises a first and a second switch, control circuitry for controlling the first and second switches, a transformer having a primary winding and a secondary winding, and a storage capacitor. The control circuitry alternatively switches the first switch ON when the second switch is OFF and switches the second switch ON when the first resonant switch is OFF, wherein the control circuitry alternatively switches the first and second resonant switches such that the ON and OFF times of each switch are substantially equal. When the first switch is ON, the storage capacitor is coupled to the internal voltage output such that the storage capacitor is charged and a current flows in a first direction through the primary winding. Alternatively, when the second switch is ON, the charged storage capacitor discharges such that a current flows in a second direction, opposite to the first direction, through the primary winding.
Because the ON and OFF times of the first and second switches are substantially equal, an output voltage produced by the secondary winding is linearly related to the internal output voltage and the current through the primary. By sensing the current through the primary and adjusting a duty cycle of the power switch accordingly, the present invention regulates the output voltage without the need for a feedback loop from the isolated secondary. In addition, the duty cycle of the power switch may be adjusted in response to directly sensing the internal output voltage through a voltage divider or the like.
In accordance with another aspect of the invention, the PWM switching power supply may be a boost converter, a buck converter, or a buck/boost converter. The storage capacitor of the alternating current tank may be a resonant capacitor either in series or in parallel with a leakage inductance of the primary, forming a series resonant tank or a parallel resonant tank, respectively.
In accordance with a still further aspect of the invention, a power converter comprising a modulated switching power supply having a power switch for regulating an internal voltage output coupled to a plurality of alternating current tanks is provided. The storage capacitor in each of the alternating current tanks may be a resonant capacitor either in series or parallel with a leakage inductance of the primary winding, forming a series resonant tank or a parallel resonant tank, respectively, as described herein. A clock coupled to the plurality of alternating current tanks permits the first and second switches to be switched synchronously with each other. The output voltage from each secondary winding is combined in parallel for application to a load. An intelligent switch may be coupled between the modulated switching power supply and the plurality of alternating current tanks wherein a given alternating current tank is coupled to the internal voltage output through the intelligent switch only when required to support a required voltage across the load.
In accordance with a still further aspect of the present invention, methods of generating DC or AC power are provided. In one embodiment, the method comprises generating an internal regulated voltage output using a modulated switching power supply. The internal regulated voltage output is coupled to a storage capacitor and a primary winding of a transformer wherein the storage capacitor is charged and a first current flows in a first direction through the primary winding during a first period. The internal regulated voltage output is then decoupled from the storage capacitor and the primary winding wherein the charged storage capacitor discharges and a second current flows in a second direction opposite the first direction through the primary winding during a second period, the first period being equal to the second period.
Other aspects and advantages of the present invention are disclosed by the following description and figures.